1. Field of the Invention
This invention relates to an improved double wall tank having an outer tank of steel and an inner tank of thermoplastic material. More particularly, this invention has an outer tank with at least two fittings thereon and the inner tank is separate and apart from the outer tank except that it is bonded to outer tank in the area of the fittings.
2. Description of the Prior Art
It is known to have outer steel tanks with inner tanks made from thermoplastic material. The inner tank is formed by rotation molding within the outer tank and separates from the outer tank when the tank cools (see U.S. Pat. No. 4,65,892). When the inner tank cools, the thermoplastic material shrinks. In tanks that have two or more fittings, as the inner tank shrinks, that portion of the inner tank wall located between the two fittings is placed under enormous stress. The stress can cause the inner tank to rupture or to fail prematurely at the corners where the inner tank bends into the flanges for the fittings.
It is also known to line a tank with an inner layer of polymeric material where the polymeric material is bonded to an inner surface of the tank (see U.S. Pat. No. 5,358,682). An advantage of a double wall tank where there is a space between the inner tank and the outer tank is that a leak detection system can monitor the space between the two walls.
It is an object of the present invention to provide an outer tank of steel having an inner tank of thermoplastic material where the outer tank has at least two fittings and the bonding of the inner tank to the outer tank can be controlled so that the inner tank is bonded to the outer tank in the area of the fittings that is otherwise separate and apart from the outer tank.
A double wall tank has an outer tank of steel and an inner tank of thermoplastic material. The outer tank has an inner surface having first areas and a second area. One of the first areas is located adjacent to each of at least two fittings that are separate from one another. The second area is a remainder of the inner surface that is not the first areas. The thermoplastic material is bonded to the outer tank in the first areas and separate from the outer tank in the second area.
A method of constructing a double wall tank having an outer tank of steel and an inner tank of thermoplastic material, said outer tank having an inner surface with first areas and a second area, one of said first areas being located adjacent to each of at least two fittings that are separate and apart from one another, said outer tank having an opening at each fitting, said second area being a remainder of said inner surface that is not said first areas, said method comprising applying bonding material to said first areas, inserting said thermoplastic material in powder form into said outer tank through an opening for a fitting, said powder including mold release material, inserting a plug into each opening, subjecting the outer tank to heat and rotating the outer tank to mold said inner tank within said outer tank, cooling said tanks, resulting in an inner wall of said thermoplastic material that is bonded to said first areas and separated from said second area, removing each plug and cutting an opening through said thermoplastic material at each opening.